Ultrasonic flow measuring devices are often utilized in the field of process- and automation technology. They permit simple determination of volume- and/or mass-flow in a pipeline.
Known ultrasonic flow measuring devices work, frequently, on the basis of the Doppler principle or on the basis of the travel-time difference principle.
In the case of the travel-time difference principle, the different travel times of ultrasonic pulses in and against the flow direction of the liquid are evaluated.
For this, ultrasonic pulses are transmitted at a certain angle to the tube axis both with and opposite to the flow. From the travel-time difference, the flow velocity, and, therewith, with knowledge of the diameter of the section of pipeline, the volume flow can be determined.
In the case of the Doppler-principle, ultrasonic waves with a certain frequency are coupled into the liquid and the ultrasonic waves reflected by the liquid are evaluated. From the frequency shift between the in-coupled and reflected waves, likewise, the flow velocity of the liquid can be determined.
Reflections occur in the liquid, however, only when small air bubbles or impurities are present, so that this principle mainly finds application in the case of contaminated liquids.
Ultrasonic waves are produced or received with the help of so-called ultrasonic transducers. For this, ultrasonic transducers are fixedly applied on the tube wall of the pipeline section of concern. More recently, also clamp-on ultrasonic flow measuring systems are obtainable. In the case of these systems, the ultrasonic transducers are, essentially, just pressed on the tube wall with a clamp. Such systems are known e.g. from EP 686 255 B1, U.S. Pat. No. 4,484,478 or U.S. Pat. No. 4,598,593.
A further ultrasonic flow measuring device, which works according to the travel-time difference principle, is known from U.S. Pat. No. 5,052,230. The travel time is ascertained, in this case, by means of short, ultrasonic pulses.
A large advantage of clamp-on ultrasonic flow measuring systems is, that they do not contact the measured medium and are placed on an already existing pipeline. Disadvantageous is a higher effort in the mounting of the clamp-on systems, in order to orient the individual ultrasonic transducers oppositely, this depending on many parameters, such as e.g. tube wall-thickness, tube diameter, velocity of sound in the measured medium. Both ultrasonic transducers of medium-contacting, inline-systems, as well as also ultrasonic transducers of clamp-on systems, require, conventionally, a relatively ample installation space outside of the measuring tube. They are, thus, exposed to mechanical influences. The ultrasonic signals between the ultrasonic transducers propagate usually on one signal path. The signal path extends, in each case, only through a fraction of the flow of the measured medium in the measuring tube, and this fact strongly influences estimation of the total flow through the measuring tube.
Ultrasonic transducers are composed, normally, of a piezoelectric element, also referred to, in short, as “piezo”, and a coupling layer, also referred to as “coupling wedge” or, not so frequently, “lead-in element”. The coupling layer is, in such case, most often, made of synthetic material, or plastic, while the piezoelectric element of industrial process measurements technology is usually of a piezoceramic. The ultrasonic waves are produced in the piezoelectric element, guided via the coupling layer to the tube wall, and, from there, conducted into the liquid. Since the sound velocities in liquids and synthetic materials differ, the ultrasonic waves are refracted at the transitions from one medium to another. The angle of refraction is determined, to a first approximation, by Snell's law. In accordance therewith, the angle of refraction depends on the ratio of the propagation velocities in the media.
Between the piezoelectric element and the coupling layer, an additional coupling layer can be arranged, a so-called adapting, or matching, layer. The adapting layer assumes the function, in such case, of transmitting the ultrasonic signal and, simultaneously, reducing reflection caused at the interfaces by different acoustic impedances of the adjoining materials.
In DE-patent 100 12 926, a sensor system for flow measurement is disclosed, which is applied fixedly on a measuring tube. This sensor system is a piezofilm sensor, which registers vibration- and/or oscillation-variables of the measuring tube, or the wall of the measuring tube, on which the piezofilm sensor is applied, and converts these vibration- and/or oscillation-variables of the measuring tube, which are produced by interactions of a medium flowing in the measuring tube with the wall of the measuring tube, into voltage signals. By means of these voltage signals, then, the flow of the measured medium through the measuring tube is ascertained.
The piezo sensor film is placed on the outside of the wall of the measuring tube, with the measured medium flowing past the inside of the wall. For protection of the piezo sensor film, also a shielding can be applied. The piezo sensor film is, preferably, polyvinylidene fluoride (PVDF). No oscillation- or vibration-signal is radiated the piezo sensor film. The piezo sensor film only receives oscillations or vibrations caused by the flow in the interior of the measuring tube. Since, now, the interactions between inside of the wall of the measuring tube and measured medium depend on many different parameters, such as e.g. composition of the measured medium and the associated Reynolds number of the measured medium, and some of these parameters change with time, e.g. due to change of the surface of the measuring tube, e.g. from abrasive wear, the accuracy of measurement of this sensor system is very limited.
U.S. Pat. No. 3,906,791 discloses an ultrasonic flow-measuring device having a measuring tube of rectangular or square cross section. In a form of embodiment, ultrasonic transducers are applied on the flat exterior of the measuring tube wall. These transmit an ultrasonic signal essentially perpendicularly to the main flow direction of the medium in the measuring tube. The ultrasonic signal is turned in the direction of the flow of the measured medium, or oppositely thereto, by recesses of triangular cross-sectional shape on the inside of the measuring tube wall.
Described in DE-application 102 04 714 is an ultrasonic transducer, which is embodied in the shape of a circular arc or a ring, and, therewith, is capable of being coupled on circularly-shaped tubes of a certain size.